Trifluralin (.alpha.,.alpha.,.alpha.-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) is a pre-emergent herbicide with little post-emergent activity. Formulations of the herbicide trifluralin are typically added to spray water at the point of use, and are applied as a fine spray to soil for the purpose of controlling undesirable weeds.
Conventionally, trifluralin formulations for use as aqueous sprays are emulsifiable concentrates. These formulations are liquids which contain a significant quantity of hydrocarbon solvent. Problems may arise from flammability, solvent contamination of the environment, and in the requirement for rigid solvent resistant storage containers which require elaborate disposal strategies.
Solid water-dispersible trifluralin formulations have been proposed, however the main problem associated with the use of solid water-dispersible trifluralin formulations is the problem of long term storage stability. Under prolonged storage, the formulations may be routinely subject to temperatures in the range 0.degree. to 40.degree. C. and may occasionally be subject to temperatures in the ranges -20.degree. to 0.degree. C. and 40.degree. to 55.degree. C. Under such storage conditions, some large (greater than 125 micron) particulate entities may form. These large particulate entities, which may account for less than 3% of the total formulation, fail to redisperse into fine particles (sub 125 micron) in the spray water, and cause blockages in the filters and nozzles of the spraying equipment. In commercial practice, such filters may have a mesh size of as little as 150 microns and it is desirable that a safety margin of at least 25 microns is achieved between the mesh size of the filter and the maximum particle size in the formulation. Filter blockages in the spraying equipment can lead to unacceptable delays in practical usage (such as down-time of major spray rigs), and even to loss in biological efficacy arising from departures from the desired rate of application. It is believed that one cause of the formation of large particulate entities may be coalescence which occurs when the trifluralin melts and freezes.
Currently available tests of long-term storage stability of trifluralin formulations are time consuming and it is frequently difficult to emulate practical storage conditions in laboratory storage tests. There is therefore a dual need. The primary need is to produce trifluralin formulations that do not fail in the field, and the secondary need is to find reliable and simple short-term tests for testing the long-term stability of these formulations.
Australian Patent No. 639678 (corresponding to U.S. application Ser. No. 301458, dated 24 Jan. 1989) discloses that solid trifluralin exists in yellow and orange polymorphic forms. The specification of this patent states that the yellow polymorph of trifluralin has higher herbicidal activity, better water dispersibility and enhanced storage stability compared to the orange polymorph of trifluralin. This specification also describes a method for producing a solid formulation comprising substantially yellow polymorph, the method involving the formation of a molten trifluralin-in-water emulsion in the presence of a water-soluble film-forming polymer such as polyvinyl alcohol-acetate. The emulsion is spray-dried to form dry product, and the yellow polymorph is stabilised by the use of crystallisation initiators such as sodium benzoate, and by the use of crystal stabilisers, such as sodium dioctyl sulfosuccinate. The thermal regime to which the newly manufactured material is subjected is stated to be critical. In particular, it is noted that rapid cooling which results in cold (-5.degree. to 5.degree. C.) powder is an efficacious method for stabilising the yellow form.
The yellow trifluralin powder made by the above process performs well in accelerated storage trials which involve heating and cooling of the formulation through the melting point of trifluralin. However, in long-term storage tests, for example, 6-12 months at ambient temperatures, it has been found that there is a gradual deterioration over time and a small quantity of orange polymorph crystals form in the formulation. This may be sufficient to block filters and nozzles in spray equipment.
In addition to the yellow and orange polymorphic forms previously mentioned, a third, amorphous (super-cooled) form of trifluralin is also known. When trifluralin is prepared in powder form it is possible (but not necessary) that trifluralin powder particles of yellow crystalline, orange crystalline, and amorphous polymorphic type co-exist in the formulation. Generally, individual powder particles are all amorphous or all yellow crystalline or all orange crystalline. Very infrequently mixed yellow crystal-orange crystal particles may be detected by microscopy. Mixed crystal-amorphous particles exist only for brief periods of time.
In a given powder sample in which different polymorphic forms of trifluralin co-exist, individual amorphous powder particles are generally randomly distributed throughout the sample. However, the situation for crystalline particles is quite different. Particles of a particular crystalline polymorphic type (yellow or orange) will generally be found in discrete macroscopic regions throughout the powder samples. For example, a substantially orange powder may have distinct yellow regions in it, and a substantially yellow powder may have distinct orange regions in it. The diameter of these regions is usually 1 mm or more and more usually 2 mm or more. The interior of the individual regions is quite homogeneous although mixed-crystal areas evidently occur along the boundaries of the regions.
Because of the colour-masking effects of various formulation additives, it is difficult to visually assess whether a particular uniform powder exists in yellow, orange or amorphous forms. However, uniform amorphous powders are easily characterised using crossed-polaroid light microscopy, and uniform crystalline powders are easily characterised by differential scanning calorimetry (DSC).
In mixed polymorphic powders, the amorphous content can be established by crossed-polaroid light microscopy and the co-existence of macroscopic yellow/orange regions is clearly apparent on visual inspection by the existence of regions or striations of a particular colour against a different coloured background. The actual colour is influenced by formulation additives.